The present invention relates to a polycarbonate resin having excellent moldability and low tendency of releasing environmental endocrine disruptors, which can be suitably used for applications such as packages for medical products and foods coming into direct contact with the human body and fluid, and for applications such as optical disks and lenses of spectacles coming into frequent contact with the human body in daily life.
Polycarbonate resins (bisphenol A polycarbonates) obtained by causing carbonate precursors to react with 2,2-bis(4-hydroxyphenyl)propane are excellent in transparency, heat resistance, mechanical characteristics and dimensional stability, finding wide applications as engineering plastics. Beside the bisphenol A polycarbonates, polycarbonate resins having various bisphenol skeletons have recently been proposed and applied to electrophotographic photosensitive materials, optical disks, heat-resistant films, polarizing films, tableware, etc.
It has recently been revealed that the polycarbonate resins thus used in every life tend to be gradually hydrolyzed to release bisphenols which are their constituents, when repeatedly placed under the conditions of high temperatures and humidity, and that free bisphenols act as pseudo-hormones of estrogens, female hormones, to link to receptors. The structural formula of estradiol, one of estrogens, and the structural formulas of compounds acting as the pseudo-hormones of estrogens to link to the receptors are shown below: 
It is known that the bisphenols tend to show a weak function as an estrogen to link to estrogen receptors. Accordingly, it may be considered undesirable to use the bisphenols likely to link to the estrogen receptors for products frequently coming into contact with the human body, though their mechanism has not been fully found.
The inventors have evaluated the properties of various bisphenols as environmental endocrine disruptors to provide polycarbonate resins having low tendency of releasing environmental endocrine disruptors. As a result, it has been discovered that extremely harmless in terms of the environmental endocrine disruptors is a bisphenol having a fluorene skeleton represented by the following general formula (I): 
wherein each of R1 to R4 is a hydrogen atom, a fluorine, chlorine or bromine atom, or an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, an alkenyl group having 2 to 5 carbon atoms, an alkoxyl group having 1 to 5 carbon atoms or an aralkyl group having 7 to 17 carbon atoms, each of which may have a substituent group bonded to a carbon atom therein, the substituent group being an alkyl or alkenyl group having 1 to 5 carbon atoms, an alkoxyl group having 1 to 5 carbon atoms, a halogen atom selected from the group consisting of the group consisting of fluorine, chlorine and bromine, or a dimethyl polysiloxy group.
The comparison in IC50 of 9,9-bis(3-methyl-4-hydroxyphenyl)fluorene, one of the bisphenols represented by the general formula (I), with other bisphenols is shown in Table 1. IC50 is defined as the concentration of an imitation required to substitute 50% of a genuine hormone linked to a receptor, and the smaller the numerical value of IC50, the higher affinity for a receptor, meaning a higher tendency of releasing an environmental endocrine disruptor.
The above table has revealed that the bisphenols having fluorene skeletons are higher in IC50 than the other bisphenols, and that they are thus highly safe compounds for humans. The polycarbonate resins having fluorene skeletons have conventionally been used for applications utilizing their optical characteristics, such as optical recording media, polarizing films, liquid crystal panels and lens. Please see Japanese Patent Laid-Open Nos. 6-145317 (1994), 6-25398 (1994), 6-216078 (1994), 6-322094 (1994) and 8-134199 (1996). Their applications for electrophotographic photosensitive materials and cast films are described in Japanese Patent Laid-Open Nos. 6-3838 (1994) and 8-134198 (1996). Further, their applications for high heat-resistant tableware usable in microwave ovens are described in Japanese Patent Laid-Open No. 6-49195 (1994), etc.
Because the polycarbonate resins having fluorene skeletons have rigid structures, it is necessary to introduce soft segments for improving their moldability. However, all of the conventional fluorene polycarbonate resins have soft segments into which constituent units such as bisphenol A having high tendency of releasing environmental endocrine disruptors are introduced. Accordingly, although the conventional fluorene polycarbonate resins contain highly safe bisphenols having the fluorene skeletons, consideration has not been sufficient on safety to humans.
It is therefore an object of the present invention to provide a polycarbonate resin exerting little influence as a environmental endocrine disrupter on the human body, even when a bisphenol or other constituent components are released therefrom by undergoing sever thermal history under the conditions of high temperature and humidity or by recycling, and excellent in moldability.
As a result of intensive investigation in view of the above object, it has been discovered that a bisphenol having a fluorene skeleton exhibits a low tendency to link to an estrogen receptor, and that by combining such a bisphenol with an aliphatic dihydroxy compound less likely to link to an estrogen receptor as a soft segment, it is possible to provide a physiologically extremely safe polycarbonate resin without impairing its properties as a polycarbonate. The present invention has been completed based on this finding.
Thus, the polycarbonate resin of the present invention comprises only constituent units derived from compounds having low tendency of releasing environmental endocrine disruptors, thereby exhibiting high safety to the human body.
In the present invention, a polycarbonate resin having useful properties can be obtained by adjusting the amount of a fluorene skeleton-containing bisphenol unit to 5 to 90 mol % based on the total constituent units.